Liquid level detection plays an important role for automated chemical analyzers and provides better control of the pipetting process. For performing pipetting operations, a pipetting needle contacts liquid contained in a vessel either for aspirating a sample thereof or for delivering a volume of another liquid to the liquid in the vessel. In order to reduce carry over and to achieve the desired accuracy of a pipetting system it is necessary to minimize contact of the pipetting needle with a vessel's content. For this purpose it is desirable to minimize the penetration depth of the tip of the pipetting needle in the liquid. Liquid level detection plays an important role for this purpose.
Most liquid level detection methods are reliable under normal circumstances but fail when operation of the pipetting systems includes piercing of a vessel's closure with the pipetting needle or when the pipetting needle encounters foam before it reaches the surface of a liquid contained in a vessel.
In the case of liquid vessels closed with a cover, usually used for storage of reagents, the level sensor of the pipetting system should be able to detect a liquid level that lies under a cover or closure (membrane, foil) of the vessel. A capacitive level sensor, widely used in chemical analyzers, does not work properly in that case and erroneously indicates detection of a liquid surface when it meets a wet cover. Capacitive liquid detectors also often erroneously detect foam lying on a liquid surface as if it were a liquid level.